> There already other extensions that do this

There are none (out of the ones I tried).

The simplest "Sort Bookmarks By Date" only works for newly created bookmarks in the default folder. If you move a bookmark to another folder via the star dropdown or drag and drop it onto another folder it will appear at the very bottom.

The same goes for "Default Bookmark Folder".

"Sort Bookmarks" has sort "by date added" but it "Does not auto-sort after moving a bookmark within a folder".

My extension doesn't mess with sorting. All it does is listening to bookmark create/move events and reposition the bookmark in the question from the last to the first position in its folder.

Tbh I moved from Opera to Firefox and tried to replicate Opera's default bookmark positioning.